Researchers at Sheffield University visited the smaller stores of Asda, Sainsbury's and Tesco in the city and discovered what harassed parents often complain of – that the queue for the checkout often passes shelves of chocolate bars, drinks and sweet snacks at children's eye level.

In a first, they analysed the food items that were at just the right height for children – about one metre from the ground – and worked out their nutritional value. The team from the university's school of health and related research found that 90% of the products children saw as their parents queued to pay were unhealthy, according to the Food Standards Agency's definition. They were packed with saturated fat, salt and sugar. The healthiest item on display was sugar-free chewing gum.

The study featured just 13 stores in Sheffield, but Dr Jason Horsley, who led it, said there was no reason to believe they were unusual. "I can't say for sure that the rest of the country is not unlike Sheffield and does not have a completely different eating pattern, but I travel around and haven't noticed any obvious differences elsewhere."

The three supermarket chains are all members of the government's public health responsibility deal, which was launched in 2011 and had been running for a year before the data for the study was collected. The deal involves food manufacturers, restaurants and supermarkets signing pledges to reduce the amount of saturated fat, salt and calories in food and drinks.

In March 2011 all three supermarket chains pledged to "support and enable our customers to eat and drink fewer calories through actions such as product/menu reformulation, reviewing portion sizes, education and information, and actions to shift the marketing mix towards lower-calorie options".

The deal is the flagship of the coalition government's anti-obesity strategy, driven by concerns particularly about the increasing weight of children, a third of whom in the UK are now overweight or obese. Overweight children are more likely to become obese adults and obesity is linked to a wide range of diseases including diabetes, heart problems and cancer.

But it is hard for parents to take a stand against the pester power of children, which has long been recognised by those involved in food marketing. "Children are a significant market for retailers of processed foodstuffs, and budgets dedicated to advertising to children have grown exponentially in the last three decades. Youngsters are often naive to sophisticated marketing techniques and they influence parents' purchases through pester power," said Horsley.

The inner-city stores were visited by medical students, who received permission from the managers to survey the items on display at child's eye height at the checkout. One store had a completely different display at the checkout from all the others: there were no sweets or junk food snacks, just dried fruit and nuts.

"The students who collected the data asked the store manager why," said Horsley. "He said because he'd seen something about a potential change in company policy or something that might be going to happen. He'd discussed it with his mum, who told him he should do it."

Some of the big chains have changed their checkout displays at larger supermarkets in response to pressure, but not at the smaller convenience stores. Sainsbury's pointed out that the sweets had long gone from the checkout lanes of their big stores, but suggested customers did not want the same thing in the smaller shops.

"We were one of the first supermarkets to stop selling confectionery at main checkouts over five years ago to help customers shopping with their families," said a spokesperson. "We also know that people shop differently in convenience stores to supermarkets and expect to find confectionery near the tills in those stores, but we always stock a good selection of fruit, nuts and other healthier alternatives alongside."

The Children's Food Campaign has been enlisting parents to pressure supermarkets to remove junk food from the checkout by handing in red cards stating "Your store has failed the checkout test", and green ones if the sweets and unhealthy snacks have been removed.

Malcolm Clark, of the campaign, said the Sheffield study was in line with international research on junk food at the tills. "Convenience stores are the next big challenge," he said.

Clark pointed out that Lidl banned sweets from the checkouts at all its 600 stores this month, replacing them with dried and fresh fruit, oatcakes and juices. Its research showed that seven out of 10 customers would choose lanes without sweets.

A Tesco spokesperson said: "Tesco was the first to take sweets off checkouts in our larger stores more than a decade ago. We are now looking at how to give customers a more balanced range of choices, including healthier products, at checkouts in our smaller convenience stores."

Asda said: "We're at a loss to understand why this narrow research is being taken seriously. Asda doesn't operate the small high street shops referred to in the study and nor does the report take into consideration the mix of food with non food at checkouts.

"We believe in offering customer's choice and encouraging healthy, balanced lifestyles. We therefore offer customers a range of different products at our checkouts including batteries, magazines as well as toiletries and some treats."